The invention is directed to a process for making, solely from abundant natural geologic material, namely shale, an ecologically acceptable liquid-impermeable barrier or liner for use in sanitary land fill operations to seal the bottom and side walls of ground cavities to prevent contamination of underground water supplies.
Because the focal point of the invention concerns fabrication of a liquid-impermeable barrier, as for land fill operations, using reconstituted shale material widely considered useless instead of the relatively expensive blue clay conventionally used for that purpose, a detailed description of the invention will be preceded by background information on:
Certain general chemical and physical characteristics of clays and shales, their origins, similarities, and differences; PA1 The conventional use of blue clay as an impermeable liner, and its growing scarcity and cost, which has created a need for a less costly but effective impermeable liner for sanitary land fill operations; PA1 The permeability coefficient by which the effectiveness of such a liner is determined; PA1 The geologic background and origin of Maquoketa Group Shale used in the present invention, and its chemical and physical characteristics to the extent known; and PA1 The impressive number of geological agencies and departments in Federal and State Governments, and universities and schools, which have overlooked the possibility of modifying shales for use as impermeable barriers and which continue to consider shales to be of little or no practical use.
Clay is a fine-grained, earthy material composed primarily of hydrous aluminum silicates. Clay type is determined by the predominant clay mineral present (that is, kaolinite, montmorillonite, illite, etc.). Any given clay sample may be predominantly one of these minerals, or a mixture of them. Clays are plastic when sufficiently wetted, rigid (but relatively soft in comparison to rocks) when dried, and vitrified when fired at a sufficiently high temperature. Clay minerals, by definition, occur in colloidal grain sizes of 2 microns or less.
Kaolinite, a hydrous aluminum silicate, Al.sub.4 (Si.sub.4 O.sub.10) (OH).sub.8 is one of the most common clay minerals and is the basic raw material for the ceramic industry.
Montmorillonite is a hydrated silicate of magnesium, the chief constituent of bentonite and fuller's earth. Bentonite has a crystalline lattice arrangement giving it ihe unusual property of expanding several times its original volume when placed in water, and has some commercial uses capitalizing on this property, as in sealing compounds for filling cracks in concrete foundations (utilizing its swelling characteristic), and pet litter (using its water absorbent characteristic). Fuller's earth has the capability of decoloring oils and fats by retaining the coloring matter and is used as a filtering agent in chemical processing.
Illite, a relatively complicated silicate of potassium, aluminum, iron, and magnesium has a crystalline lattice arrangement giving it a mica-like character. Prior to this invention, which uses high illite shale, illite has had little, if any, known utility, although it is abundantly present in clays and shales.
Shale, sometimes called "mud rock", has been defined (W. H. Twenhofel, Principles of Sedimentation, Second Edition, McGraw Hill Book Company) as a minutely-grained sedimentary rock which is layered or laminated parallel to the bedding and in which there has been no substantial change in mineral composition since accumulation other than changes that resulted from chemical changes in place or compaction. Shales differ from clay in that they are bedded or laminated; they generally contain quartz, which imparts a rock-like hardness, making them incapable of being readily plasticized by kneading with water. In some cases, nearly identical minerals have been classified, one as clay and another as shale, solely on the basis of induration (hardness). Thus, the first observation concerning clays and shales, significant to the present invention, is that, as deposited in the earth, clays and shales are chemically similar, often differing only in physical character, mainly hardness.
Clay has been used as an impermeable lining material for wells and cisterns since Biblical times. The particular clay which has been used for this purpose is believed to be the so-called "blue clay", mostly kaolinite. It has been used to line wells and cisterns both in the as-mined, plastic state, and in the form of oven-fired bricks. The raw plastic clay and bricks have sometimes been combined, such as by use of a layer of plastic clay between the excavation wall and an inner, protective brick liner. As another application, clay dams or stoppings have been used in early underground coal mines to divert mine water from the working areas. In still another application, blue clay has been used as a water-impermeable liner on the inner faces of earth-filled dams. Blue clay, thus, has acquired the status of an approved impermeable lining material, much in demand, and costly. In the Chicago, Ill. area, for example, the cost of locally produced blue clay delivered to the site is approximately $4 per cubic yard. As will be explained, to line the bottom and side walls of a typical 200 foot deep, 50-acre excavation with blue clay to a thickness of 10 feet would cost almost $5 million for the lining material alone. As blue clay supplies are depleted close to the point of need, and haulage distances and costs increase, the cost of blue clay for this kind of large scale use will become prohibitive.
As specified in "Procedures for Testing Soils", 1964, The American Society for Testing and Materials has devised standard permeability tests from which the permeability coefficient of a particular lining material may be determined. One criterion in determining the ecological acceptability of a lining material is that it have a permeability coefficient not greater than 5 .times. 10.sup..sup.-8 centimeters per second as determined by the falling head permeability test specified in the ASTM standards. A 10 foot thick lining of material having this low permeability coefficient would, for example, delay seepage from garbage and refuse of a sanitary landfill site for so long that it would be rendered completely harmless by exposure and age.
The geological and stratigraphic sequences pertinent to the present invention are as follows: The geologic time scale beginning about 600 million years ago is divided into Paleozoic (ancient), Mesozoic (middle) and Cenozoic (recent) Eras. The present invention is directed to use of shales dating from the Paleozoic Era, which itself is divided into seven periods. Beginning with the earliest, the Paleozoic periods are designated Cambrian, Ordovician, Silurian, Devonian, Mississippian (lower carboniferous), Pennsylvanian (upper carboniferous) and Permian. Of these, the present invention relates to the last part of the Ordovician Period, which commenced about 500 million years ago and extended for 75 million years. The Ordovician Period is subdivided into three epochs, the Canadian, the Champlainian, and the Cincinnatian which was the most recent. The Cincinnatian geologic series, deposited during the Cincinnatian Epoch, includes, at the very bottom, Cape Limestone. Above the Cape Limestone, and continuing to the top of the Cincinnatian Series, is the "Maquoketa Shale Group", from which the preferred shale formation used in the present invention is taken.
The Maquoketa Shale Group is named for exposures on the little Maquoketa River in Dubuque County, Iowa. It underlies vast sections of the midwestern United States, and frequently constitutes the initial geologic layer beneath the valuable deposits of commercial Niagarian and Alexandrian dolemite and limestone which are used for highway and building construction.
Using the State of Illinois as a specific example, the Illinois Geological Survey indicates that the Maquoketa Shale Group consists of a lower unit, the Scales Shale, overlain by a middle limestone (the Ft. Atkinson Limestone), and an upper shale (the Brainard Shale). This pattern, while generally accurate, is subject to local and regional variations. For example, in Northern Illinois, an additional member of the Maquoketa Group, the Neda Formation (largely red shale interbedded with red-brown or black hematitic oolite) is locally present as the uppermost member of the group. In extreme Southwestern Illinois, to illustrate the variations caused by the general westerly uplift of the strata, only the generally lowermost segment of the group, Scales Shale, is found.
In the present invention, whereas the entire Maquoketa Shale Group is of general interest, the Brainard Shale formation, generally the uppermost member of the Group, which has been found to immediately underlie the commercial dolemite beds of Northern Illinois, is of specific interest. Prior to the present invention, there has been no known use for Brainard Shale, first, because it is too brittle and thinly laminated to support loads and, second, it readily deteriorates into dust when subjected to wet and dry weathering cycles as evidenced in the specific areas where it is exposed.
Of great significance to the discovery of useful properties of the Brainard Shale is the fortuitous geological coincidence whereby it lies directly beneath much of the commercially quarried Silurian dolomite and limestone formations. It is 75-100 feet thick where it is not deeply truncated by the sub-Silurian unconformity. It is greenish gray to green, is itself partly dolomitic in some areas, and locally silty. The Brainard Shale is commonly fossiliferous and it is identified by the presence of Cornulite fossils in its upper reaches. The Cornulite fossils are unique, having been deposited only at the close of the Ordovician Period, and serve as a tag to identify the Maquoketa Shale Group though it has sometimes been identified by other names in areas other than the central Midwest. Most important, Brainard Shale is vast, thick, and composed of almost pure illite clay particles. Published data (Illinois State Geologic Survey, Report of Investigations No. 203, 1957) indicates the clay fraction (-2 micron) of this shale is generally 90%-100% illite.
Because knowledge of the stratigraphic sequence of underground formations is of great value in the search for and development of metals, minerals, and energy resources, much detailed information has been developed and is widely known through investigations and publications of geologists and engineers associated with the U.S. Geological Survey, the Geological Surveys of the individual states, and the many state universities and colleges having geology, mining, ceramic, and engineering schools.
Despite this respectable array of talent and the very substantial sums of money which have been spent by various agencies on earth studies and evaluations of study results to identify commercially important minerals, prior to the present invention no one has developed a method for reconstituting and utilizing shales as an inexpensive substitute for clay, even considering the need and the widespread availability of these shales, and considering especially that millions of dollars may be saved on a single sanitary land fill operation by using a reconstituted shale barrier according to the present invention instead of a lining of conventional blue clay.